The invention relates to a diffractive security element as set forth in the classifying portion of claim 1.
Such diffractive security elements are used for verifying the authenticity of a document and are distinguished by an optically variable pattern which changes in a striking and predetermined fashion from the point of view of the person observing it by virtue of rotation or tilting movement.
Diffractive security elements of that kind are known from many sources, reference is made here as representative examples to EP 0 105 099 B1, EP 0 330 738 B1 and EP 0 375 833 B1. They are distinguished by the brilliance of the patterns and the movement effect in the pattern, they are embedded in a thin laminate of plastic material and they are glued in the form of a stamp onto documents such as bank notes, bonds, personal identity papers, passports, visas, identity cards as set forth. Materials which can be used for production of the security elements are summarized in EP 0 201 323 B1.
Modern color photocopiers and scanner devices are capable of duplicating such a document in apparently true colors. The diffractive security elements are also copied, in which case admittedly the brilliance and the movement effect are lost so that the pattern which is visible in the original at a single predetermined angle of view is reproduced as an image with the printing colors of the color photocopier. Such copies of documents can be confused with the original under poor lighting conditions or if the observer is not paying attention.
When dealing with colored surface portions which are arranged in side-by-side relationship the human eye perceives a color contrast if the wavelengths of the spectral colors in the surface portions differ by fewer than ten nanometers (nm). Particularly in the range of between 470 nm and 640 nm an observer still notices differences of between 1 nm and 2 nm (W. D. Wright & F. G. G Pitt “Hue discrimination in normal colour vision”, Proc. Physical Society (London) Vol. 46, page 459 (1934)).
A known idea, which is based on the differences in spectral sensitivity of the human eye and the color photocopier, is that of providing documents with a colored background and printing the information on the background in a different color, in which case the information, in relation to the background, involves a contrast which is perceptible by the human eye but which cannot reproduced by the color photocopiers.
EP 0 281 350 B1 discloses such a colored security paper which as a background has a repetitive, for example check, pattern consisting of two colors A and B, wherein the information is printed onto the background pattern, in a further color S. The spectral reflectivities of the colors A, B and S are so selected that the color photocopier can admittedly recognize and reproduce a contrast between A and S in regions of the color A, but not between B and S in regions of the color B. Therefore, of the information, only the parts which are in the regions A are visible on the copy.
U.S. Pat. No. 5,338,066 discloses two methods of distinguishing a colored original produced by a printing procedure from its colored copy. The one method detects color constituents of the printing ink and the other method is based on the different dynamic regions in image processing of the color photocopier compared with image processing of the human eye. In the original, the colors of the background and the information involve a modulation of ±5% in respect of their spectral reflectivity, wherein the background color has the maximum in the green spectral range of visible light and the color for the information has respective maxima in the blue and red spectral ranges. The spectral reflectivities of both complementary colors are of the same value, averaged over the visible spectral range, and together form the color white. While the eye can easily perceive the crimson information against the green background the color photocopier only registers a white to slightly gray surface.
The reference to additional chemical detection for detecting the authenticity of an assumed original involves the technique of color mixing, which in practice is difficult to manage.